Standard optical fibres confine light in a region of the fibre known as the core. The core region is generally placed in the centre of the fibre cross section. In recent years, fibres known as “multicore” fibres (MCFs) have been developed for various applications including sensing and lasers. In contrast to standard fibres, MCFs contain numerous cores that are situated throughout the fibre cross section. Depending on the application, the core regions may be separated by as little as a few microns and can be arranged in either a one dimensional or two dimensional array.
Due to the core geometries of MCFs, the coupling of light into and out of each core can pose a significant problem. In the case of MCFs consisting of a one-dimensional array of cores, the coupling issues, although challenging, can be overcome through the use of known optical devices. However, significantly greater problems currently exist when coupling light to and from MCFs that have a two dimensional array of cores.
Currently, only three methods are known to potentially allow direct MCF to single core fibre coupling. The first is a theoretical proposal described by S. B. Poole and J. D. Love [Electron Lett., 27, 1559-1560 (1991)]. In this, the ends of two lengths of single core fibre are inserted into a hollow capillary. This capillary is then heated and tapered down to enclose the fibre ends within. By correctly controlling the fabrication, the separation of the fibre cores at the end of the taper can be controlled to match the core geometry of the MCF. As a result, light from the MCF can be directly coupled from the tapered end of the standard single core fibres to the MCF. A problem with this is that correctly shaped hollow capillaries have to be used to place the fibres, which limits the device that could be fabricated.
The second known method to allow direct MCF to single core fibre coupling involves creating a device by etching standard optical fibres using hydrofluoric acid, and then attempting to arrange them into the same spatial geometry as the core geometry of the MCF by using a frame or capillary into which the fibres are inserted, as described by G. M. H. Flockhart et al [Opt. Lett. 28, 387-389 (2003)]. A problem with this is that the fibres are extremely fragile after etching and the method involves the use of toxic substances. Furthermore, the core geometries for which a suitable device could be fabricated are limited since correctly shaped hollow capillaries have to be used to place the fibres.
The third technique involves fusion splicing a single core fibre directly to an MCF. Coupling between the single core fibre and the MCF is then achieved by tapering the fibre in the region of the fusion splice using a heat source, as described by L. Yuan et al [Appl. Optics. 47, 3307-3312 (2008)]. This technique is therefore only suitable currently for simultaneously addressing all cores of an MCF with a single core fibre and involves the fabrication of a taper, which may be extremely fragile.